In The Coworker by Freida McFadden, you can always tell when Dawn is in the office because she arrives he same time every day and has no friends. On day she is not at her desk. Natalie, her co-worker, gets a phone call that changes everything.
Freida McFadden's books are very good and this is no exception. It has plenty of twists and turns. Good luck guessing how this one all plays out! (Gerard's review, 5++ stars)